1. Field of Invention
This invention is related to noise reduction in video images. More particularly, this invention is directed to a system and method for reducing white noise in images.
2. Description of Related Art
Images, such as still images captured from video signals, can sometimes be quite noisy. This noise generally falls into one of two categories. The first category is xe2x80x9csalt and pepperxe2x80x9d noise. Salt and pepper noise is occasional, high-frequency light or dark pixels caused by electric pulses, such as from nearby engines or motors. The application of a median filter to images containing salt and pepper noise works well to clean up the noisy pixels in the image.
The second category of noise is white, or Gaussian, noise. White noise is a random noise throughout the image and is commonly seen in the xe2x80x9csnowyxe2x80x9d pictures from distant broadcast stations. Approaches to reducing white noise have been either through the application of a Wiener filter, or through averaging over several frames.
While salt and pepper and white noise each have filters that work well to clean-up the pixel errors in images, these filtering approaches may not always be practical. Specifically, the design of an optimal Wiener filter requires knowledge of the communication channel characteristics and of the particular noise statistics. Averaging images, which is a common approach to reducing white noise, requires the availability of several originally identical images. In video signals, this would require that no motion occur in the captured image. Neither of these approaches are possible if only a single image from an unknown source is available.
This invention provides systems and methods for reducing noise in images.
This invention separately provides systems and methods for reducing white noise in still images.
This invention separately provides systems and methods for decomposing a still image into a plurality of frequency bands and recomposing the still image from the filtered frequency bands.
This invention separately provides systems and methods for setting filtering parameters for a frequency band of a still image.
The various systems and methods of this invention reduce white noise by first applying band-pass filters to the image to decompose it into a sequence of frequency bands. A different threshold is then applied to each band. Small changes in the image are considered to be noise and are removed, while large changes are retained as the signal. The various threshold bands are then reassembled to produce the resultant image having reduced white noise content.
These and other features and advantages of this invention are described in or are apparent from the following detailed description of the preferred embodiments.